1. Field of the Invention
The present invention relates to a confocal displacement sensor and, more particularly, to improvement of a confocal displacement sensor that measures displacement of a measurement object using a confocal optical system.
2. Description of Related Art
A confocal displacement sensor is an optical measurement device that measures displacement of a measurement object making use of a confocal principle of stopping down received light into reflected light from an image forming surface on which an image of a light source is formed and a phenomenon of an axial chromatic aberration in which a color drift in an optical axis direction occurs in the image of the light source.
The confocal displacement sensor is configured by a pinhole that emits, as a point light source, light emitted from a light source, an optical member that causes an axial chromatic aberration in detection light emitted via the pinhole and converges the detection light toward the measurement object, and a spectroscope that spectrally disperses reflected light from the measurement object. As the detection light, light having a plurality of wavelengths, for example, white light is used. The pinhole allows, in the detection light irradiated on the measurement object via the optical member, detection light having a wavelength reflected while focusing on the measurement object to pass.
The position of the image forming surface is different for each wavelength according to the axial chromatic aberration. Therefore, displacement of the measurement object is calculated by specifying a wavelength of the detection light passed through the pinhole. The displacement is the distance in the optical axis direction from a predetermined reference position to the measurement object. The depth or the height of unevenness on a surface, the thickness of a transparent body, and the like can be measured by calculating the displacement.
When a rough surface body that irregularly reflects light with fine unevenness of the surface thereof is used as the measurement object, detection light focused on the surface is reflected in random directions. Therefore, an intensity level after spectral dispersion decreases and an error increases. In particular, a measurement value sometimes greatly fluctuates exceeding a degree of surface roughness depending on a measurement part. Therefore, for example, accuracy is low when displacements are respectively measured in a plurality of measurement parts using a plurality of displacement gauges and flatness of the measurement object is calculated from measurement values.
When the position of a displacement gauge slightly deviates, a measurement value greatly fluctuates because of the influence of irregular reflection. Therefore, the measurement value is unstable depending on a setting state of the displacement gauge.